Ok, so i had this problem and now it's SOLVED!
For some time now, after i have upgraded my Iphone 2G to the 2.2.1 version, my iphone kept giving me an error that goes something like "this accessory is not made to work with iphone"...
After some 2-3 weeks of this error at least once a day, at some moment i forgot to charge my iphone and the battery went empty after night. In the morning it have noticed it can't charge, can't reset, can't go into DFU, etc. I have tried all the tricks found on numerous forums but nothing.
What i did to solve this problem is:
1. I have opened my iphone using a cutter and a screwdriver. You can find info on how to open your iphone on google.
2. i took out the batery nicely cutting the three wires at the midle (black=minus, red=plus, white=ground).
3. Following the instructions at
http://dailycupoftech.com/2008/07/21/usb-pinout/
i took two wires from the usb of my computer (V BUS (5V) and Ground) and joined them with the wires from the battery of my Iphone. (USB 5V+ to Battery plus / USB Ground to battery minus). You could use i guess any charger that outputs DC 5V, but i didn't had any in hand.
4. I left the battery charging for about two hours.
5. I deconected the batery from the usb "charger" and joined the wires back on the Iphone, as they first were. ! be carefully at the exact order of the wires, and don't short-chircuit the wires together or touch the Iphone board with them. Isolate them with duct tape or whatever you have handy. (i have used paper

i know ... )
6. I put the Iphone back together.
7. My Iphone booted after pressing the upper right power button at first try. It can charge and is detected by the computer, i can restart it, DFU, whatever.
8. I have upgraded my Iphone to 3.0 as the problem seemed to be related to some update/unlock procedure for 2.2.1.
In the past 3 days my Iphone haven't complained about anything, no errors, no "this accessory is not made to work with iphone", it seem to work just fine.
BE VERRY CAREFOULY if you decide to do this, but I think it is worth a try as otherwise your Iphone will remain a paper weight anyway.
Good luck.
sory for my bad english